As I'm walkin' through this mentally, yes and no. There are several sort of back-story events where it's not. Ditto the friendship journey for one character that has another character seducing her. However, the one main repeatable event between the heroine and one of her school friends certainly is; it's entirely optional. There's not a lot of lesbian content overall, I'd have to say, although there are a number of MFF threesomes.